The distribution of heme-containing respiratory pigments capable of reversibly binding oxygen is well documented throughout the animal kingdom. Work in our laboratory has shown that at least two species of the ciliate protozoan paramecium, contain such respiratory pigments. One of the species, P. tetraurelia has been shown to contain five electrophoretic variants. These multiple forms appear to be identical in molecular weight and amino acid composition and by spectral analysis although differing in their isoelectric points. The present project is concerned with an analysis of the functional aspects of each of the five molecular forms as well as attempts to isolate and characterize the myoglobin from a second ciliate species - P. multimicronucleatum.